Dielectric Insulation Gasket for a Vacuum Bottle

ABSTRACT

The gasket makes it possible for the apparatus, in particular the vacuum bottle, to be assembled and disassembled relatively easily and conveniently. It imparts high dielectric performance to the apparatus. The inside and the outside contact surfaces of the gasket are smooth, and, for example, each of them can be made up of two cylindrical portions ( 31 A &amp;  31 B;  32 A &amp;  32 B) of different concinnities. At least one of the side surfaces has an annular recess ( 35 ). Application to switchgear using vacuum bottles for operating at high and medium voltages.

FIELD OF THE INVENTION

The invention relates to the field of electrical equipment and installations, and in particular switches and switchgear using vacuum “bottles” operating at medium and high voltages.

A particular use is for overhead transport of electricity.

PRIOR ART AND PROBLEM POSED

In electrical installations and switchgear, switches use vacuum bottles that must be capable of withstanding stresses, in particular dielectric stresses, between the contacts situated inside the bottle, in the vacuum, and also between the external ends of the bottle disposed in ambient air. With a view to making the dielectric strength uniform between live contacts and external ends of vacuum switches, and in view of the compactness required, it is necessary to use insulating elements other than the air outside the vacuum bottles themselves.

Thought has been given in particular to dielectric solid or fluid insulators, such as the greenhouse gas sulfur hexafluoride (SF₆). Insulating vacuum bottles in air does not make it possible to obtain suitable dielectric performance with small dimensions.

However, insulating vacuum bottles in dielectric gaseous fluids such as SF₆ is costly. It is necessary to use a gastight tank equipped with feedthrough bushings, which is highly detrimental to the environment, in particular as regards pollution, recycling, and the greenhouse effect.

Solid insulation systems for insulating vacuum bottles are highly temperature-sensitive and they cannot be disassembled or dismantled at the end of their lives when they have stuck or bonded together. That therefore has consequences that are highly detrimental to the environment.

With a view to reducing the impact on the environment, it has been proposed to use a combined insulator using both a solid insulator and a gaseous fluid insulator, such as air at atmospheric pressure or some other gases such as nitrogen. In which case, the solid insulator is of small volume because it is implemented in the form of a gasket having a gas-proofing function and a dielectric function. However, in systems known from the prior art, that type of insulation does not make it possible to obtain high dielectric performance for vacuum bottles.

With reference to FIG. 1, a vacuum bottle 101 is surrounded at both ends of its outside surface with two gaskets 102A and 102B. The top gasket 102A is placed in the vicinity of the stationary contact of the vacuum bottle 101, whereas the bottom gasket 102B is placed in the vicinity of the moving contact. The resulting assembly is placed inside a rigid shell 103 made of an insulating material. Unfortunately, the structure of the gaskets 102A and 102B is such that air is trapped at their surfaces 104 that come into contact with the inside wall of the rigid shell 103.

FIG. 2A is a fragmentary section view of the surface referenced 104 in FIG. 1. Said surface is made up of a plurality of lips 105 of pointed section, separated from one another by gaps 106.

FIG. 2B is also a fragmentary section view showing the same place on the prior art gasket. Said gasket has been inserted into the rigid shell 103, and its lips are thus flattened, or rather folded slightly, all in the same direction, by the pressure from the inside wall of the shell 103 on one side of each lip 105. Air is thus trapped between each lip 105 along a line B-B′. Similarly, air can be trapped along the line A-A′, on the other surface. That air, which is of low dielectric strength, considerably limits the dielectric performance of the system. A striking arc can easily move radially within each interface gap 106 in order to seek to weakest point on the circumference of the next lip 105 and thus propagate to the next gap 106. The overall dielectric strength is a function of the sum of the weakest points on each circumference of the gasket 102. In addition, the thickness of insulator at certain places around the gasket 102 is too small to obtain high dielectric performance. Finally, the shape of the prior art gasket 102 is not very favorable to disassembly or dismantlement at the end of its life because of the non-return or “check” effect of the lips 105.

An object of the invention is thus to obtain high dielectric performance with small dimensions for vacuum bottles by acting on their insulation, in particular by preventing the tracking of electrical discharges or sparks along the contact surfaces of the gasket in service. In addition, it is desired to comply with environmental constraints. Full and easy dismantling of the insulation system at the end of its life is thus desired. Furthermore, it is proposed to use a smaller amount of solid insulating material. This contributes to reducing cost, compared with an entirely solid insulation system.

European Patent Application EP 1 017 142 A1 describes a circuit-breaker switch having a combined insulation system.

SUMMARY OF THE INVENTION

To this end, the invention mainly provides a dielectric insulation gasket for a vacuum bottle, said gasket being designed to insulate a vacuum bottle by using at least one gasket around the vacuum bottle inside a casing, each gasket having an inside contact surface and an outside contact surface, two side surfaces interconnecting the inside and the outside contact surfaces.

According to the invention, an inside contact surface of the casing and an outside surface of the of the vacuum bottle being smooth, the inside and outside contact surfaces of the gasket are smooth, presenting no cavity and forming part of a group constituted by surface shapes comprising surfaces that are convex relative to the longitudinal axis of the gasket and surfaces presenting a gradient that does not reverse relative to the longitudinal axis of the gasket. Thus, when assembling the gasket, no gas pockets are trapped in the interfaces, neither between the inside contact surface of the casing and the outside contact surface of the gasket nor between the outside surface of the vacuum bottle and the inside surface of the gasket, thereby removing any risk of partial electrical discharges appearing between, firstly the inside contact surface of the casing and the outside contact surface of the gasket and secondly between the outside contact surface of the vacuum bottle and the inside contact surface of the gasket.

This resistance to tracking is characterized by the ability of the gasket to fit perfectly against the outside surface of the vacuum bottle or the inside face of the casing to oppose the formation of electrical sparks which would carbonize the surface of the gasket and/or the outside surface of the vacuum bottle or the inside face of the casing, and would thus provide a path for current flow.

A main embodiment makes provision for said inside contact surface and said outside contact surface to be cylindrical.

A second main embodiment makes provision for said inside contact surface and said outside contact surface to be conical.

A third main embodiment of the inside and outside contact surfaces of the gasket is that each of said surfaces is made up of two conical portions of different concinnities and interconnected via a determined interconnection curve forming a flared V-shape.

In the two preceding embodiments, it should be noted that it is preferable for the general directions of the inside and outside surfaces to be conical and of opposite concinnities relative to each other.

Concerning the general structure of the gasket, it is also preferable for the width of the inside and outside contact surfaces to be equal to or greater than 5 millimeters (mm) in order to limit the risks of arcs striking or tracking at said interfaces.

It is particularly advantages for the minimum thickness of the gasket along the longitudinal axis of the gasket to be at least 4 mm. These two provisions make it possible to increase the dielectric strength of the gasket considerably.

In various embodiments that are provided, the gasket has a recess in its cross-section, so as to limit the forces in the gasket.

As regards the side surfaces, in order to control thermal expansion, provision is also made for the side surfaces to be in two portions having different inclinations.

Provision is also made for at least one of the side surfaces to be rounded, the other being straight.

Provision is also made for the side portions to be rounded in part, one portion being concave, and another portion being convex.

Provision is also made for the gasket to have a trapezium-shaped section, i.e. an outside contact surface and an inside contact surface that are parallel to the axis of revolution of the gasket, the side surfaces being inclined in opposite directions.

The cross-section of the gasket may be H-shaped.

The cross-section of the gasket may also be N-shaped.

It may also be M-shaped.

It may also be square or rectangular in shape.

When the cross-section of the gasket is provided with a recess, it may be W-shaped or U-shaped.

LIST OF FIGURES

The invention and its various technical characteristics will be understood more clearly on reading the following description, illustrated by various figures, in which:

FIG. 1, described above, is a view showing the use of two prior art gaskets;

FIGS. 2A and 2B are fragmentary section views showing the active portion of a prior art gasket;

FIG. 3A is a section view showing the use of a gasket of the invention;

FIG. 3B is a section view showing the use of two gaskets of the invention;

FIGS. 4A to 4D are detail views showing four embodiments of gaskets of the invention; and

FIGS. 5A to 5M are section views of various gaskets of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As shown in FIGS. 3A and 3B, a vacuum bottle 1 is placed in a casing 10 constituting the pole of medium-voltage or high-voltage electrical switchgear. In this example, the casing 10 thus constitutes the rigid pole of switchgear used, such as a circuit-breaker. When the circuit-breaker is in the open position, a moving contact 5A of the vacuum bottle 1 and a stationary contact 5B, each of which is placed at a respective end of a vacuum bottle 1, are at different electrical potentials. It is thus necessary to insulate the vacuum bottle 1 dielectrically by placing a gasket 20 constituting a dielectric insulation gasket between the moving contact 5A and the stationary contact 5B constituting two electrodes of different potentials. Once in place, the gasket 20 prevents tracking of sparks or discharges along the dashed-line lines A-A′ and B-B′. For information, it is indicated that the dielectric strength of the vacuum inside the vacuum bottle 1 is significantly greater than the dielectric strength of the air outside the vacuum bottle 1.

With reference to FIG. 3B, when two gaskets are used, said gaskets isolate an annular space 24 defined by a side surface of each of the gaskets 20, by an outside surface 6 of the vacuum bottle 1 and by an inside surface 16 of the casing 10. The space 24 that is confined in this way contains a gaseous fluid, such as air or some other fluid of the same type. In other words, the two gaskets 20 and the space 24 that they define form a dielectric barrier between the moving contact 5A and the stationary contact that are at different potentials. This configuration makes it possible to avoid any arc striking through the dielectric or bypassing one of the gaseous elements defined by the gaskets 20, either by tracking, or by perforation. More precisely, the dielectric sealing or dielectric strength is provided, inter alia, by three elements, namely:

the intimate contact between the gaskets 20 and the vacuum bottle 1, in particular via its outside surface 6, along the line A-A′ and at the contacts between the gaskets 20 and the casing 10, in particular via its inside surface 16, along the line B-B′;

the radial compression of the gaskets 20 which are made of an elastomer material; and

the correctly dimensioned thickness of the insulating elastomer material of each gasket 20.

To this end, it can be observed that, in the embodiment described in FIG. 3B, each gasket 20 has a section made up of two conical portions 20A and 20B inclined in opposite directions. In other words, the section of said gasket is approximately U-shaped. This is merely a relatively simple example of a shape for the gasket, other more elaborate shapes being described in the following paragraphs.

A very important technical feature of the gasket of the invention is that the peripheral outside surface and the peripheral inside surface of each gasket 20 are smooth. A casing 10 is used whose inside surface 16 is smooth, and, similarly, the vacuum bottle 1 has an outside surface 6 that is smooth. The inside and the outside surfaces of each gasket 20 are correspondingly smooth. Air is thus prevented from being trapped between the surfaces during assembly. The general shape of the gasket is optimized, so as to obtain contact pressures at the gasket/casing and gasket/vacuum bottle interfaces that are not uniform, but that are sufficient. The tightness with which the gasket clamps around the vacuum bottle 1 is greater than the tightness with which the gasket is clamped by the casing 10. This enables the gasket to remain in place on the vacuum bottle during assembly, disassembly, and dismantling.

As can be observed in FIG. 3B, the positions of the gaskets on the vacuum bottle 1 are optimized in that said gaskets are positioned on said vacuum bottle in zones in which the dielectric fields are favorable to high dielectric strengths. In particular, said gaskets 20 are not in contact with the electrodes constituted by the moving contact 5A and by the stationary contact 5B. Otherwise, a major risk of the gaskets being perforated exists in the event that a local electric field that is too strong appears. A projection on one of the electrodes would give rise to an electric field concentration. Should a dielectric sealing gasket be in contact with one of said electrodes, said electrode would be subjected to the electric field that is too strong, and could be degraded by perforation.

FIG. 4A shows a first embodiment of the gasket in detail.

The outside contact surface, which is smooth, is actually made up of two surfaces 31A and 31B, both of which are conical relative to the axis 30 of the gasket, their inclinations being different, so as to form an outwardly very open U-shape. They are interconnected via an outside interconnection curve RE. In analogous manner, the inside contact surface is made up of two portions 32A and 32B, each of which has a different inclination relative to the axis 30, it being possible for one of them (the surface 32A in this example) to be cylindrical. The two inside contact surfaces are also interconnected, via an inside interconnection curve RI. The interconnection curves RE and RI contribute to preventing air from being trapped while the gasket is being mounted. Although the gaskets 20 are shown mounted around the vacuum bottle 1 and in the casing 10 with smooth contact surfaces, it should be emphasized that said outside and inside surfaces are smooth when the gaskets are not mounted.

In this embodiment, the two side surfaces are also made up of a plurality of portions. One of them is provided with a recess 35 constituted by two frustoconical surfaces 35A interconnected via a radial surface 35B. Said recess 35 makes it possible to limit the forces within the gasket, when said gasket is compressed, while the vacuum bottle is being assembled into the casing.

Similarly, the other side surface is made up of two surfaces 33A and 33B, which are themselves frustoconical, and of different inclinations so as to form a very open U-shape. The remainder of the side surfaces is constituted by radial portions, firstly 34C, and secondly 34A & 34B that connect the recess 35 to the inside contact surfaces.

The shape in this embodiment is similar to a U-shape whose vertical portions extend downwards slightly. Other possible sections for the gasket, in particular letter-shaped sections, are described below.

Regardless of the shape considered, the thickness in the direction parallel to the axis 30 of the gasket must be equal to or greater than 4 (four) millimeters. The mechanical strength is thus naturally reinforced, but it is, above all, the dielectric strength of the gasket that is thus increased, in particular by considerably limiting the risks of an arc striking by perforating the gasket.

Similarly, if the inside contact surfaces 32A & 32B and the outside contact surfaces are of sufficiently large axial height, constituting bearing surfaces extending over large areas and not merely localized bearing surfaces, they contribute above all to increasing the dielectric strength of the gasket. An axial height of at least 5 (five) millimeters is thus required. It should also be noted that the electric fields at the interface constituted by the inside contact surfaces 32A and 32B, and by the outside surface of the vacuum bottle are higher than the electric fields at the interface constituted by the outside contact surfaces 31A and 31B and by the inside surface of the casing. The width of the inside contact surfaces 31A and 31B is thus greater than the width of the outside contact surfaces 32A and 32B. For the same clamping pressure during assembly, disassembly, and dismantlement, this enables the gasket to remain in place on the vacuum bottle.

The gasket is made of an elastomer material. While it is being mounted, it being deformed makes it possible to obtain contact pressures that are sufficient at its inside contact surfaces 32A and 32B and at its outside contact surfaces 31A and 31B. The system is insensitive to temperature. By means of the shape of its side surfaces, the gasket is free to expand when the temperature rises, and to contract when the temperature falls.

The ratio of the areas subjected to pressure, i.e. the inside contact surfaces 32A and 32B and the outside contact surfaces 31A and 31B, to the areas that are free, i.e. the side surfaces 33A, 33B, 34A, 34B, 35A, and 35B is sufficiently small for the elastomer material of which the gaskets are made to expand and to contract freely with variations in temperature. This makes it possible to limit considerably the thermo-mechanical stresses within the gasket. Depending on the ratio of the loaded areas to the free areas, said thermo-mechanical stresses can degrade the systems.

Such a gasket has been qualified on an application of nominal voltage of 38 kV. It is capable of withstanding IEC and ANSI standardized voltages: a withstand voltage of 95 kilovolts root mean square (kVrms) for 60 seconds (s) at a frequency of 50 hertz (Hz), and a lightning strike voltage of 200 kVc with partial discharges less than or equal to 5 pico coulombs (pC). It withstands temperatures in the range −40° C. to +115° C. continuously.

Other detailed embodiments are shown in detail in FIGS. 4B, 4C, and 4D.

FIG. 4B shows an embodiment of the gasket that has a general shape similar to the shape shown in FIG. 4A, except that the outside contact surface 41 and the inside contact surface 42 are cylindrical and parallel to the axis 40 of the gasket. This gasket also has a recess 45 opening out on a side surface completed by two side surface portions 44A and 44B. The other side surface is constituted by a portion 44C perpendicularly connecting to the inside contact surface 42.

FIG. 4C shows an embodiment of the gasket with a conical outside surface 51 and a conical inside surface 52, sloping in opposite inclinations. The remaining portions of the side surfaces are of design similar to the preceding side surfaces, i.e. one side surface has a recess 55 completed by two side portions 54A and 54B, the other side surface being completed by a side portion 54C.

Finally, a fourth embodiment is shown in FIG. 4D, in which the outside contact surface 61 and the inside contact surface 62 are curved with a relatively large radius of curvature. It can be observed that the general directions of the two surfaces are inclined slightly relative to the axis 60 of the gasket, i.e. they have frustoconical general directions that are opposite from one surface to the other. This type of gasket also has a side recess 65 completed by two side portions 64A and 64B, the other side surface being completed by a side portion 64C.

FIGS. 5A to 5M show that it is possible to give the gasket a section that is different from the section described in FIG. 4. The section shown by FIG. 5A is a rectangle. In other words, the side surfaces are perpendicular to the axis 50, whereas the inside contact surface and the outside contact surface are parallel thereto.

In analogous manner, FIG. 5B shows a gasket section that is square.

The section shown in FIG. 5C is trapezium-shaped, the inside and the outside contact surfaces still being concentric with the axis 50, but the side surfaces having respectively opposite inclinations.

The section shown in FIG. 5D has side surfaces constituted by two portions of opposite inclinations relative to the perpendicular to the axis 50, i.e. forming surfaces that are slightly convex.

The section shown by FIG. 5E presents a side surface that is perpendicular to the axis 50, and a rounded side surface of convex shape.

The section shown by FIG. 5F presents side surfaces in two portions, having different and opposite inclinations, forming a convex side surface that is V-shaped and a concave side surface that is V-shaped.

FIG. 5G shows a gasket one of whose side surfaces is made up of two surfaces of opposite inclinations forming a convex side surface, while its other side surface is slightly rounded.

FIG. 5H shows a gasket each of whose side surfaces is made up of two portions, and more precisely, has a concave portion and a convex portion, the side surfaces being S-shaped.

The section shown by FIG. 5I is an H-section, a recess of quadrilateral shape being formed in each side surface.

FIG. 5J shows a U-shaped section.

FIG. 5K shows a W-shaped gasket section.

FIG. 5L shows an M-shaped gasket section.

Finally, FIG. 5M shows an N-shaped section.

ADVANTAGES OF THE INVENTION

The dielectric performance of switchgear equipped with such gaskets is relatively high for switchgear that is relatively compact.

The dielectric strength is high at the contact interfaces between the gasket and the casing and between the gasket and the vacuum bottle.

Similarly, inside the gasket, the dielectric strength is high.

This resistance to tracking is characterized by the ability of the gasket to fit perfectly against the outside surface of the vacuum bottle or the inside face of the casing to oppose the formation of electrical sparks which would carbonize the surface of the gasket and/or the outside surface of the vacuum bottle between A and A′ and/or the inside face of the casing between B and B′ (FIGS. 3A and 3B), and would thus provide a path for current flow either between A and A′ or between B and B′.

The switchgear is relatively easy to dismantle at the end of its life, and the quantities of insulating material are small, complying with environmental standards.

This solution is of relatively low cost, and it is easy to industrialize by means of mass-production molding at high throughput and by means of adhesive-free assembly.

The assembly is insensitive to temperature variations, the gaskets being free to expand or to contract.

Assembly is easy because the gasket is easy to deform.

Finally, the system is dismantlable. 

1. A dielectric insulation gasket for a vacuum bottle, said gasket being designed to insulate a vacuum bottle by using at least one gasket around the vacuum bottle inside a casing, the gasket having an inside contact surface and an outside contact surface, two side surfaces interconnecting the inside and the outside contact surfaces; said gasket being characterized in that an inside contact surface of the casing and an outside surface of the of the vacuum bottle being smooth, the inside contact surfaces and outside contact surfaces of the gasket are smooth, presenting no cavity and forming part of a group constituted by surface shapes comprising surfaces that are convex relative to the longitudinal axis of the gasket and the surfaces presenting a gradient that does not reverse relative to the longitudinal axis of the gasket, in such a manner, that when assembling the gasket, no gas pockets are trapped in the interfaces, neither between the inside contact surface of the casing and the outside contact surface of the gasket nor between the outside surface of the vacuum bottle and the inside surface of the gasket, thereby removing any risk of partial electrical discharges appearing between, firstly the inside contact surface of the casing and the outside contact surface of the gasket and secondly between the outside contact surface of the vacuum bottle and the inside contact surface of the gasket.
 2. A gasket according to claim 1, characterized in that the axial height of the inside contact surface and of the outside contact surface is equal to or greater than 5 mm.
 3. A gasket according to claim 1, characterized in that the minimum thickness of the gasket along the longitudinal axis of the gasket is 4 mm.
 4. A gasket according to claim 1, characterized in that the inside contact surface and the outside contact surface are cylindrical relative to the gasket axis.
 5. A gasket according to claim 1, characterized in that the outside contact surface and the inside contact surface are conical.
 6. A gasket according to claim 1, characterized in that the inside contact surface and the outside contact surface are each made up of two conical portions having different inclinations and interconnected via a determined interconnection curve (RE, RI) forming a flared V-shape.
 7. A gasket according to claim 5, characterized in that the inside contact surface and the outside contact surface are of general direction inclined relative to the axis of the gasket, i.e. of conical general direction.
 8. A gasket according to claim 1, characterized in that its cross-section is provided with a recess.
 9. A gasket according to claim 1, characterized in that each of the side surfaces is made up of a convex portion and of a concave portion.
 10. A gasket according to claim 1, characterized in that the cross-section of the gasket is H-shaped.
 11. A gasket according to claim 1, characterized in that the side surfaces are made up of two portions of different inclinations.
 12. A gasket according to claim 1, characterized in that at least one side surface is rounded.
 13. A gasket according to claim 1, characterized in that the cross-section of the gasket is square in shape.
 14. A gasket according to claim 2, characterized in that the cross-section of the gasket is rectangular in shape.
 15. A gasket according to claim 1, characterized in the cross-section of the gasket is trapezium-shaped.
 16. A gasket according to claim 1, characterized in that its cross-section is N-shaped.
 17. A gasket according to claim 1, characterized in that its cross-section is M-shaped.
 18. A gasket according to claim 1, characterized in that its cross-section is U-shaped.
 19. A gasket according to claim 1, characterized in that its cross-section is W-shaped.
 20. A gasket according to claim 6, characterized in that the inside contact surface and the outside contact surface are of general direction inclined relative to the axis of the gasket, i.e. of conical general direction. 